Disposal of agricultural-based waste such as food processing by-products, perishable items and products with fixed term shelf life as well as food waste from institutional facilities and restaurants is a significant problem. Solid organic agricultural bio-waste typically ends up in landfill sites while liquids are often disposed of via the local sewage systems. Waste oil products, such as yellow and brown grease, are most troublesome as they clog sewage systems and end up in natural water ways to detrimentally harm the ecosystem. In particular, Canada, with a population of approximately 35 million, specifically produces approximately 140,000 tonnes of waste cooking oil (WCO) each year primarily from the fast-food and restaurant industries. The transesterification of oils to form esters, particularly methyl esters, has received considerable attention as an environmentally-friendly way of recycling and disposing of waste oil into biodiesel.
Biodiesel is a fuel derived from biologically sourced fatty acids such as fatty acid glycerides or fatty acid esters from lipid containing plant material, microbes or animals. It is a mono-alkyl ester derived from the processing of organic oils like vegetable oils and alcohols. Processing is typically carried out by an esterification reaction mechanism and is normally performed in an excess of alcohol to maximize conversion. Esterification may occur directly between a fatty acid and an alcohol, or via transesterification, such as between an ester and an alcohol. While vegetable oils and alcohols are the most common reactants in the esterification process, any source of fatty acid, such as free fatty acids, soaps, esters, lipids, glycerides, amides and monohydric alcohols may also be esterified, as well as be employed in various combinations as reagents in the esterification reaction.
Biodiesel, also known as fatty acid methyl esters (FAME), is produced through a transesterification process where waste oil and methanol are reacted in the presence of a catalyst; this reaction is described in FIG. 1.
At present, there are several different processes involving catalysis that may be used to generate biodiesel from waste oils. The processes differ from each other based on the type of catalyst used, and are generally classified as follows:
1. Homogeneous basic catalyst;
2. Homogeneous acidic catalyst;
3. Heterogeneous basic catalyst; and
4. Heterogeneous acidic catalyst.
Homogeneous Basic Catalyst
The oldest method to produce biodiesel is through the use of a strong, liquid-formed, basic catalyst. The advantages with this process are the rapid transesterification reaction and cheap cost of the catalyst. However, this catalyst suffers from a serious limitation. The total free fatty acid (FFA) content of the lipid feedstock must not exceed 0.5 wt %, otherwise soap is produced as a by-product which requires extra units and steps for its removal, which subsequently increases the production cost of biodiesel. Other disadvantages include: difficulty in separating the liquid catalyst from the product; the catalyst's danger to operators due to its basic nature; and the fact that the catalyst cannot be recycled. Since the catalyst can only be used once, it leaves the process as waste; fresh catalyst must be continually added for the process to continue. Common homogeneous basic catalysts used are sodium hydroxide (NaOH) and potassium hydroxide (KOH). Noureddini (U.S. Pat. No. 6,174,501), Hammond, E. et al., (U.S. Pat. No. 6,965,044), Khalil, C. et al. (U.S. Pat. No. 7,112,229), and Woods, R. et al. (US 2008/0209799) utilize homogenous basic catalysis for the production of biodiesel.
Homogeneous Acidic Catalyst
Another popular choice for biodiesel production is to utilize a homogeneous acidic catalyst as it is cheap to purchase. Although the transesterification reaction using this catalyst is considerably slower compared to using liquid basic catalyst, this can be remedied if more methanol is added though it may increase the production cost. On the other hand, this may be a fair trade-off since biodiesel production using acidic catalyst does not produce soap as a by-product despite the FFA content of the lipid feedstock. The homogeneous acidic catalyst shares the disadvantages of the liquid basic catalyst-the catalyst is difficult to separate from the product, it poses a danger to the operators, and it cannot be reused in the reaction. This leads to a problem with waste generation as well as a large need for fresh catalyst. The most commonly used and efficient liquid acidic catalyst is sulfuric acid (H2SO4).
Heterogeneous Basic Catalyst
In order to reduce the amount of waste generated from used (i.e. spent) liquid basic catalysts, heterogeneous alkaline catalysts were developed as they can be recycled back into the production process. Additional advantages of this type of catalyst are that it can be easily separated from the product, and it is generally less harmful to handle than a homogeneous catalyst. The main disadvantages are the catalyst's high cost and the extremely slow reaction rate. Although the addition of methanol to the reaction can improve the reaction speed, the reaction process will still continue to run slower than when using either acidic or basic homogeneous catalysts. Another factor to consider is the catalyst's reusability as heterogeneous catalysts tend to degrade and lose their activity from leeching during the process. Several examples of a heterogeneous basic catalyst are magnesium oxide (MgO), calcium methoxide Ca(CH3O)2, and zinc oxide (ZnO). Lin, V. et al. (US 2008/0021232) discloses methods of preparing such catalysts for the production of biodiesel.
Heterogeneous Acidic Catalyst
Similar to the heterogeneous basic catalyst, heterogeneous acidic catalysts are safer to human health than their homogeneous counterparts as they are less corrosive. In addition, heterogeneous acidic catalysts result in a reduction of waste as such catalysts can be recycled, but unfortunately leeching and catalyst degradation may also occur. Other disadvantages are that production and recovery costs for this type of catalyst tend to run very high, which is a common trait shared among most heterogeneous catalysts. The rate of production of biodiesel is also decreased compared to using liquid catalyst as well. Some popular heterogeneous acidic catalysts that are used include Amberlyst-15, Nafion and zeolites. Several different methods utilizing heterogeneous acidic catalysis for the production of biodiesel have been disclosed by Fleisher, C. (U.S. Pat. No. 7,420,072), Boocock, D. (U.S. Pat. No. 6,642,399, U.S. Pat. No. 6,712,867, EP 1,206,437 B9), Jackam, J. et al. (EP 1,889, 889 A1), and Horton, C. (WO 2007/113530). While employing such catalysis, these alternate methods are typically limited by use of harsh chemicals and solvents in their reaction and purification processes.
Recently, a potential new, organic source heterogeneous acidic catalyst has been identified-sugar. Sugar catalyst is a promising option for esterification reactions due to its advantages pertaining to its organic nature, cost, re-usability and effectiveness. Toda et al. (Nature, 2005), Okamura et al. (Chem. Mater. 2006) and Zong et al. (Green Chem. 2007) disclose methods of producing such carbon-based catalysts by sulphonating an incomplete carbonization of D-glucose, creating a robust, re-usable catalyst that that does not lose its activity from leaching. Because it is non-reactive it is safe, as well as being non-toxic, environmentally friendly and relatively inexpensive. The major disadvantage with a sugar catalyst is that the transesterification reaction rate is lower than using homogeneous acidic catalysts. Zong et al. (Green Chem. 2007) also report it is one of the most efficient catalysts allowing 97% conversion of waste oil to biodiesel with only a 10:1 methanol to oil ratio. It has the added advantage of being able to be recycled up to 50 times, thus making the reaction process more cost efficient as the catalyst does not have to be replaced frequently.